A primary concern for an opto-electrical transmitter such as a vertical cavity surface emitting laser (VCSEL) is control of the light intensity entering an optical fiber from the opto-electrical transmitter. A driver for the opto-electrical transmitter typically controls a drive current to the opto-electrical transmitter to control the emitted light intensity. The drive current generally has a level set according to a data signal and the performance characteristics of the opto-electrical transmitter, but for safety, the driver current should be limited so that the light intensity entering the optical fiber does not exceed eye safety levels.
FIG. 1 shows a conventional driver 100 that controls the output power level of a transmitter 110. As illustrated in FIG. 1, driver 100 provides a current having two components Ibias and Imod. Current Imod is modulated between 0 and a maximum current IMOD according to the logic value of a data signal DATA and maintains an average value of IMOD/2 under normal operating conditions. Current Ibias controls the current level for a logic value 0 (i.e., when Imod=0) and contributes to the average current level (Ibias+IMOD/2), which controls the average emitted light intensity and has a level controlled by an analog feedback loop in driver 100.
For the feedback loop, driver 100 includes a monitor diode 120, an amplifier 130, and a current source 140. Monitor diode 120 is a photodiode that generates a voltage Vmon according to an intercepted portion of the light emitted from transmitter 110. Monitor diode 120 is a slow device (relative to variations in a data signal DATA) so that voltage Vmon is directly related to the average intensity of the light emitted from transmitter 110. Alternatively, monitor diode 120 could comprise a xe2x80x9cfast xe2x80x9d device with the addition of a low-pass-filter such that Vmon is related to the average intensity of the light emitted from transmitter 110.
Amplifier 130 is a differential amplifier having a negative input terminal coupled to monitor diode 120, a positive terminal connected to receive a reference voltage REF, and an output terminal coupled to control the bias current Ibias through current source 140. Accordingly, amplifier 130 is in a feedback loop that limits the average intensity of light emitted from transmitter 110. In particular, when the light intensity increases, monitor voltage Vmon increases causing amplifier 130 to reduce bias voltage Vbias and causing current source 140 to reduce bias current Ibias. When the light intensity decreases, monitor voltage Vmon decreases causing amplifier 130 to increase bias voltage Vbias and causing current source 140 to increase bias current Ibias.
The feedback loop drives bias current Ibias towards an equilibrium level that depends on reference voltage REF. In turn, the average intensity of emitted light depends on the current Ibias+IMOD/2 and the performance of transmitter 110 in converting current to emitted light. In the embodiment of FIG. 1, a calibration process for transmitter 110 selects the resistance of a resistor 132 and thereby selects reference voltage REF according to the performance of transmitter 110 and monitor diode 120. The setting of the resistance 132 compensates for permanent or structural variations between opto-electrical transmitters such as a variation in VCSEL efficiency and the feedback loop can compensate for temporary variations in the operation of transmitter 110.
Driver 100 also requires a mechanism to shut down transmitter 110 in the event of a permanent unsafe condition. A permanent unsafe condition can arise, for example, when a short in monitor diode 120 to ground causes the feedback loop to increase the drive current so that the average emitted intensity remains above the eye safety level.
Driver 100 includes a mechanism to shut down transmitter 110 if monitor voltage Vmon rises to a level indicating that the output power of light from transmitter 110 is unsafe. In particular, a differential amplifier 150 compares monitor voltage Vmon to a maximum voltage MAX. A resistor 152 has a resistance selected to set maximum voltage MAX at the appropriate level according to the fraction of the light monitor diode 120 receives and an eye safety level for the total intensity. If monitor voltage Vmon rises above maximum voltage MAX, the output voltage from amplifier 150 sets a latch 154, which in turn shuts off the currents Imod and Ibias via switches 160 and 161 such that no current flows to transmitter 110.
A disadvantage of driver 100 is that driver 100 shuts down and becomes inoperable as soon as the voltage Vmon from monitor diode 120 rises above voltage MAX. However, if the unsafe condition is transient, e.g., if transmitter 100 is functioning properly but some external transient effect caused the laser power to temporarily rise above the eye safety level, driver 100 shuts down, breaking any communication link through transmitter 110. Generally, it would be desirable not to assert a fault signal so that the communication link can remain intact, as a transient high light-output-level is eye-safe as long as the transient time is short.
Another disadvantage of driver 100 is the requirement of analog components such as resistors 132 and 152 that must be calibrated according to the specific performance of transmitter 110. The analog components are difficult to fabricate in a small device package. Additionally, the analog time constant may make such analog circuits unable to meet all timing requirements.
A digital system can overcome many of the drawbacks of analog drive circuits. U.S. pat. No. 5,019,769 describes a digital laser drive system that uses a digital data processor. While this digital system avoids many of the drawbacks of analog drivers, the requirement of a digital data processor increases the complexity and cost of the driver. Accordingly, a digital driver circuit is sought that avoids the drawbacks of analog driver circuits and distinguishes between permanent and temporary unsafe conditions but does not require the cost or complexity of a digital data processor.
In accordance with an aspect of the invention, an opto-electrical transmitter such as a VCSEL has a driver with a digital feedback loop and digital fault detection. The fault detection allows the driver and opto-electrical transmitter to continue operating in the event where the average light intensity exceeds the maximum eye-safe level allowable for continuous exposure. Henceforth, this maximum allowed continuous exposure level will be referred to as CESL, for continuous emission safe level. The digital feedback loop includes an up/down counter having an output count that controls the bias current for the opto-electrical transmitter. In response to a clock signal, the counter counts up or down if a monitor current from a monitor diode indicates the average emitted light intensity is less than or greater than a desired intensity. If the count reaches a maximum (overflow) or minimum (underflow) value, a fault condition is detected.
Additional fault detection circuitry in the driver includes a second counter. The second counter counts up if the monitor current indicates the average power of the emitted light is outside a target range and counts down or resets if the monitor current indicates the average emitted light intensity is in the target range. If the count from the second counter reaches a trigger level a fault is detected. Accordingly, if the laser power temporarily exceeds the CESL but returns to a level below the CESL before the second count reaches the fault trigger, the fault signal is not activated and the driver can continue to operate. But, if the laser power is persistently outside the CESL, the second count will reach the fault threshold, and the fault signal is activated.
One specific embodiment of the invention is a driver circuit for an opto-electrical transmitter. The driver circuit includes a monitor diode, a counter, an output driver, and a fault activation circuit. The counter is connected in a feedback loop with the monitor diode and the output driver. In particular, the counter counts up or down depending on the power output that the monitor diode measures for the opto-electrical transmitter, and the output driver provides to the opto-electrical transmitter a bias current at a level depending on a count from the counter. The fault activation circuit is connected to activate a fault signal in response to the count from the counter reaching a trigger value. The trigger value can be a value that overflows or underflows the counter or a value that overflows or underflows the input range of a digital-to-analog converter that converts the count to a control signal for the output driver.
Another embodiment of the invention is also a driver circuit for an opto-electrical transmitter. This embodiment includes a counter, a comparator circuit, and a fault activation circuit. The comparator circuit provides an enable signal and a reset signal to the counter. For example, the comparator circuit enables the counter in response to a digital parameter of the driver circuit being outside a target range and resets the counter when the digital parameter is in the target range. The fault activation circuit activates a fault signal that disables operation of the driver circuit in response to a count in the counter reaching a trigger value. Accordingly, the counter times the duration of each interval that the digital parameter is outside the target range. The digital parameter can be a parameter that controls a bias current output from the driver circuit to the opto-electrical transmitter or a parameter that corresponds to power that a monitor diode measures for the output of the opto-electrical transmitter. A memory can store high and low margin values that the comparator circuit uses when determining whether the digital parameter is in the target range. Accordingly, the target range is easily programmable according to the performance of an opto-electrical transmitter.
Optionally, this embodiment of the invention further includes a digital-to-analog converter that converts the digital parameter into an analog voltage that controls the bias current output from the driver circuit. In such an embodiment, a second fault activation circuit can activate the fault signal in response to the digital parameter being outside a range of proper input values for the digital-to-analog converter.
Another variation of this embodiment further includes a monitor diode and a second counter. The second counter has the digital parameter as a count value and is connected so that the second counter counts up or down depending on a value from the monitor diode indicating the power output from the opto-electrical transmitter. A second fault activation circuit activates the fault signal in response to the digital parameter being outside a second range. The second range can extend between the limits of proper input values for a digital-to-analog converter that converts the digital parameter into an analog voltage for control of the bias current output from the driver circuit or between the overflow and underflow values of the second counter.
Yet another embodiment of the invention is a method for controlling operation of a drive circuit for an opto-electrical transmitter. This method includes timing a period during which a digital operating parameter of the drive circuit is outside a target range, and activating a fault signal in response to the period extending beyond a maximum period. Timing the period can be conducted by determining whether the digital operating parameter of the drive circuit is within the target range and incrementing a counter in synchronization with a clock signal as long as the digital operating parameter is outside the target range. Generally, the counter is reset when the digital operating parameter is within the target range, and the fault signal is activated if the count reaches a trigger value corresponding to the maximum period. The digital parameter can be a value that controls a bias current output from the driver circuit to the opto-electrical transmitter or a value that corresponds to power measured for light output of the opto-electrical transmitter.